The invention deals with a process for the non-galvanic tin plating of copper or copper alloys by precipitating tin from a tin-containing electrolyte, consisting of methanesulphonic acid and a complexing agent.
Non-galvanic tin precipitation is known from the current state of the art and is commonly used, based both on acidic and alkaline electrolytes. Primarily, copper and copper alloys are tin plated in an ion exchange process, for example pipes, pipe sections and fittings for cold and hot water, battery posts, sanitary connectors as well as conductor frames. As the source of the tin for the electrolytes especially bivalent tin salt is used, such as for example tin chloride, tin sulfate, tin tetrafluorborate or tin methanesulphonate.
The formation of non-galvanically precipitated tin layers on copper and copper alloys is effected by the exchange of copper for tin atoms, whereby the removal of the copper is made possible by a complexing agent.
A generic process is described in DE 197 49 382 A1. The process described there refers to the tin plating of pipes, pipe sections and fittings of copper or a copper alloy by the chemical precipitation of a tin layer. Methanesulphonic acid, tin methanesulphonate, a complexing agent as well as a wetting agent are suggested as electrolyte.
The tin layers produced with the tin precipitation processes known heretofore only grow until no more surface copper can pass through the porous tin layer. The achievable layer thickness is therefore limited to a maximum of 2 xcexcm. The disadvantage is that a diffusion of metals from the base material, especially of alloy components, can occur which may lead to undesirable effects. For example, copper of a potable water pipe may dissolve and diffuse through the tin and can enter the water, which may have effects detrimental to health. Also, the liberation of lead and zinc from brass base materials, for example, can not be prevented by the precipitation of a generic tin layer. In addition, difficulties with soldering of the surface of tin plated base materials due to the diffusion are a disadvantage.
In order to avoid the above disadvantages it is the purpose of the invention to devise a process for the non-galvanic tin plating of copper or copper alloys by which a durable tin layer which can easily be soldered can be produced which, at the same time, prevents the liberation of the basic material.
As a solution it is proposed by the invention that at least one foreign metal is added to the electrolyte to form a diffusion barrier in the tin layer.
With the process described in the invention, a tin bath is suggested for the formation of a tin layer by chemical precipitation, which contains at least one foreign metal. The addition of foreign metals to the tin bath achieves an advantageous suppression of the diffusion processes, and thus a diffusion barrier is built which prevents the liberation of metals from the base material to a large extent. The advantages thus gained are good soldering characteristics at the surface and good durability of the tin layer.
The formation of a tin layer by the above process therefore not only creates the possibility to produce effective corrosion protection but, moreover, by the use of foreign metals a diffusion-stable tin layer is produced which prevents the liberation of materials from the base layer to a large extent. This is an advantage especially in view of the copper liberation from potable water carrying copper tubing. However, an out-diffusion of lead and zinc from basic brass materials is prevented by a diffusion-stable tin layer.
In accordance with one feature of the invention, a metal of the group silver, bismuth, nickel, titanium, zirconium and indium is suggested as the foreign metal, whereby the use of indium has shown to be especially effective. For the formation of a diffusion barrier within the tin layer, at least one of the above metals is added to the tin bath as a foreign metal.
In accordance with an additional feature of the invention, thiourea and/or its derivative is used as the complexing agent. Thiourea as the complexing agent enables the liberation of positively charged copper ions. A copper thiourea complex forms which is soluble in electrolytes at a temperature of  greater than 28xc2x0 C. As a result of the complexing of the copper, its potential compared to that of tin is reduced. The then more noble tin precipitates, forming a layer of tin on the copper. The liberated copper ions concentrate in the electrolyte, whereby at a copper concentration above 7 g/l economical working is no longer possible since at these concentrations tin is no longer precipitated at satisfactory rates. It is therefore suggested to remove the copper by precipitation of the copper-thiourea compounds in solution in the electrolyte. In this manner, a substantial increase in the useful life of the tin bath may be achieved. The precipitation of the copper-thiourea compounds can be achieved by means of another feature of the invention by filtration.
For the application of the non-galvanic precipitation of diffusion-stable tin layers in accordance with the invention a tin bath is of advantage, which preferably contains the following components:
1. A source of tin,
preferably a bivalent tin salt, for example tin methanesulfonate, with 1 to 30 g/l of tin in the tin bath;
2. An acid,
preferably methanesulphonic acid with 5 to 200 g/l in the tin bath,
whereby the tin bath assumes a pH value of 0 to 3;
3. A complexing agent,
preferably thiourea or a derivative in quantities of 10 to 200 g/l;
4. A wetting agent in quantities of 1 to 10 g/l;
5. At least one foreign metal,
preferably a metal in the group Ag, Bi, Ni, Ti, Zr and In in a proportion of 1 to 500 mg/l in the tin bath.
To apply the process described in the invention, a working temperature of the tin bath of 35 to 80xc2x0 C. is suggested. In addition, already known measures common to the state of the art can be taken when using the process described. This includes, for example, rinsing, pickling and drying of the work pieces.